The use of textiles as thermal insulators has become increasingly important during the last few years. Textiles differ from other solid material in that they have an air volume, enclosed in fiber spaces, in addition to the solid fiber material. In general, air is known to be a very good thermal insulator. The air content in a textile, which is usually high, provides a very good thermal insulation effect. However, the good thermal insulation effect of textiles is lost to a considerable degree, if for instance the air volume decreases as the textile is being compressed, if the textile absorbs water or vapor from the environment, or if it is heated. Since climatic conditions of the environment, i.e. temperature as well as humidity, may frequently change during the use of a textile as a thermal insulator, and since in addition the material is often compressed during processing, knowing the thermophysical characteristics of textiles under various anticipated application conditions is a prerequisite for its appropriate use and continued assurance of the required thermal insulation effect.
However, in the known test procedures for the determination of the thermophysical characteristics of textile materials, the stated requirements are hardly met. In most instances, the thermophysical characteristics are determined only for material temperatures within the range of the room temperature. In this test procedure, the material samples are heated on one side, and the temperature on the backside of the material is kept constant at a value corresponding to the room temperature. Most of the known measuring procedures are based on stationary measuring principles, in other words, the actual measurement is taken only when a constant thermal flow through the material sample is achieved, i.e. a constant temperature difference exists between the two sample surfaces. Setting this stationary state of equilibrium can take several hours. During this long-term heat application, the moisture content of material, which was wet or damp before measurements began, may change considerably. Therefore, these known procedures are suitable only for investigations of dry material samples.
One essential disadvantage of a number of known single-plate methods is that the material sample is placed onto the heating plate without any special attachment, so that frequently an air cushion may be formed between the heating plate and the material sample, resulting in false measuring results. In the known two-plate method, the material sample is arranged between two plates, one of which functions as aheat source and the other functions as a heat sink. The upper of the two plates exerts vertical contact pressure on the material sample which can result in an essential decrease in material thickness, for example in loosely structured samples. However, the resulting change in material thickness is not considered in the determination of thermophysical characteristics, which again leads to considerable measuring errors.
In addition, the contact pressure cannot be varied in all known measuring procedures for the determination of thermophysical characteristics of textile materials, so that it is impossible to conduct appropriate practical investigations. Another disadvantage of the known measuring procedures is that measurements cannot be carried out on very thin samples, since it is impossible to achieve a stationary temperature difference between the two sample surfaces. Therefore, materials with thicknesses in a range of millimeters are usually investigated in several layer packs with this procedure, resulting again in increased measuring errors, since air cushions may form between the material layers.